DECLARE @XML XML
DECLARE @NEWVALUE DATETIMEOFFSET

set @XML='<ScheduleDefinition xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
  <StartDateTime xmlns="http://schemas.microsoft.com/sqlserver/2006/03/15/reporting/reportingservices">2013-12-30T04:30:00.000+00:00</StartDateTime>
    <WeeklyRecurrence xmlns="http://schemas.microsoft.com/sqlserver/2006/03/15/reporting/reportingservices">
    <WeeksInterval>1</WeeksInterval>
    <DaysOfWeek>
      <Sunday>true</Sunday>
      <Monday>true</Monday>
      <Tuesday>true</Tuesday>
      <Wednesday>true</Wednesday>
      <Thursday>true</Thursday>
      <Friday>true</Friday>
      <Saturday>true</Saturday>
    </DaysOfWeek>
  </WeeklyRecurrence>
</ScheduleDefinition>'

;WITH XMLNAMESPACES('http://schemas.microsoft.com/sqlserver/2006/03/15/reporting/reportingservices' AS X)
SELECT @NEWVALUE = TableAlias.FieldAlias.value('(X:StartDateTime/text())[1]', 'DATETIMEOFFSET') FROM @XML.nodes('//ScheduleDefinition') AS TableAlias(FieldAlias)

SET @NEWVALUE = DATEADD(MINUTE,2,@NEWVALUE)
DECLARE @FORMAT VARCHAR(max) = CONVERT(VARCHAR(MAX),@NEWVALUE,126)

SET @XML.modify('
			 declare namespace x="http://schemas.microsoft.com/sqlserver/2006/03/15/reporting/reportingservices";
			 replace value of (/ScheduleDefinition/x:StartDateTime[1]/text())[1]
			 with sql:variable("@FORMAT")
			 ')
SELECT @XML

 